Many different medicament delivery devices have been developed and designed which have been provided with automatic functions such as automatic dose delivery when activated. The automatic functions are in many cases performed by energy members such as springs that are in a tensioned state before activation.
For many medicament delivery devices, compression springs have been used to a large extent. Some of the advantages with compression springs are the reliable performance and the low manufacturing cost. Some of the disadvantages are that the devices tend to be long and that force delivered is reduced when the spring is extended, leading to possible problems to deliver enough force through the whole stroke. Therefore, in order to ascertain enough force, the springs are often designed such that the initial force is very high, usually unnecessarily high. This provides additional problems especially when the spring is pre-tensioned during assembly of the medicament delivery device because the high built-in forces from the spring may cause material deformation, which in turn may cause breakage or mal-function of the medicament delivery device.
Some developers have then turned to other types of energy members because of the drawbacks with compression springs. One type of energy members that have been considered is a type of flat spiral spring, in the form of a steel band wound to a package. The inner end is then often attached to a drive member of the medicament delivery device while the outer end is attached to a component that can be fixed in relation to the housing. In order to pre-tension the spring, the package is tightened by turning the outer end of the spring in relation to the inner end of the spring, or vice versa.
This type of spring provides much less built-in force when pre-tensioned and also the built-in forces have a better force distribution in the device than the compression springs. One of the devices that utilize the above mentioned type of spring is disclosed in document WO 2009/037141. However, one aspect that may be regarded as a drawback with this type of spring is that the diameter of the device tends to be larger than a more conventional pen-type medicament delivery device. This is mainly due to the spring package and also that the outer end of the spring has to be attached to a housing component that obviously has to be outside the spring as seen in a radial direction, thereby adding to the thickness of the medicament delivery device.